1. Field of the Invention
The present invention relates to an ac generator for a vehicle such as a truck, a passenger car or the like.
2. Description of the Related Art
Power of vehicle generators has been increased to operate various control devices, while the size of the vehicle generators has to be kept as small as possible in order to be disposed in an engine compartment that contains those of the various control devices.
In order to increase the output power of ac the generator, the number of magnetic poles of the vehicle ac generator has been increased from 12 to 14 or 16. This change does not increase production cost very much, because such ac generators having different number of magnetic poles can use many common parts and or components.
It is known that the output power of an ac generator is controlled according to rotation speed of the rotor. The rotation speed is detected by a rotation speed detector from voltage induced in a phase winding of the ac generator by residual magnetism. The phase voltage is converted into a pulse signal having a frequency, from which the rotation speed is counted.
If the number of magnetic poles changes, a ratio of the frequency to the rotation speed also changes. Therefore, it is impossible to use a conventional rotation detector to ac generators having different number of magnetic poles.
It is a main object of the invention to provide a vehicle ac generator that has an improved rotation speed detecting device that can be adjusted according to the number of magnetic poles when or after it is assembled.
According to a feature of the invention, an ac generator for a vehicle includes a controller having means for detecting rotation speed of a rotor according to frequency of voltage induced in a phase-winding. The controller includes a frequency-pulse conversion circuit for providing a pulse signal having a plurality of times as many frequencies as the voltage induced in the phase-winding. The detecting means detects the rotation speed based on the pulse signal. The frequency-pulse conversion circuit can be applied to at least two kinds of ac generator that has different number of magnetic poles to detect the rotation speed.
Preferably, the pulse signal has 7 times as many frequencies as the voltage induced in the phase-winding. The frequency-pulse conversion circuit of the ac generator is composed of a wave-shaping section and a pulse multiplying section. Therefore, the voltage is converted into a rectangular wave voltage signal having a basic frequency, which is converted into a pulse signal having a predetermined number of times as many pulses as the basic frequency. For that purpose, the pulse multiplying section is composed of 6 serial pulse increase stages each of which adds one pulse formed from a leading edge of a pulse of a pulse signal inputted thereto that delays by a predetermined time, which is an ordinal number of the stagexc3x97the delay time.
The pulse signal may have 4 times as many frequencies as the voltage induced in the phase-winding. In this case, the pulse multiplying section is composed of 3 serial pulse increase stages each of which increases one pulse formed from a leading edge of a pulse of a pulse signal inputted thereto that delays by a predetermined delay time, which is an ordinal number of the stagexc3x97the delay time. In this case, three pulse increase stages can be applied either to an ac generator having six pairs of magnetic poles or to another ac generator having eight pairs of magnetic poles if one pulse increase stage is invalidated. Thus, it is not necessary to change the rotation speed detecting means.
The frequency-pulse conversion circuit of the above controller is composed of a clock for generating a basic clock signal and a plurality of serially-connected frequency dividing circuits. The basic clock signal has such a frequency that the output signal of the final increase stage can have a shorter cycle than a half of a maximum cycle of the voltage induced in the phase winding. Therefore, the pulse multiplying section can operate accurately because the increased pulses do not overlap each other.
The frequency-pulse conversion circuit provides the pulse signal having a pulse cycle that is determined according to a multiplying command signal sent from the rotation speed detecting means. The rotation speed detecting means determines the pulse cycle according to the rotation speed of the rotor that is calculated from a frequency of the pulse signal inputted thereto so that the pulse cycle can become shorter as the rotation speed of the rotor is higher and longer as the rotation speed is lower. This improves noise proof characteristic of the circuit.
According to another feature of the invention, a method of manufacturing two kinds of ac generators each of which has a controller that is composed of a rotation speed detecting means and a frequency-pulse conversion circuit. The method includes a step of forming the frequency-pulse conversion circuit for a larger number P1 of pairs of magnetic poles on an IC chip, a step of changing the frequency-pulse conversion circuit to a modified frequency-pulse conversion circuit for a smaller number P2 of pairs of magnetic poles on an IC chip by invalidating connection of a portion of the frequency-pulse conversion circuit by changing a portion of the IC chip, thereby changing N1 to N2 that is P1xc3x97N1/P2. The rotation speed detecting means may be formed on an IC chip that is different from the frequency-pulse conversion circuit so that the IC chip on which the rotation speed detecting means is formed can be used for different kinds of ac generators without change. However, the rotation speed detecting means may be integrated into the IC chip together with said frequency-pulse conversion circuit. Preferably, P1xc3x97N1 or P2xc3x97P2 is the least common multiple of P1 and P2.
In the above method, the frequency-pulse conversion circuit preferably includes a wave-shaping section, a pulse multiplying section, and a pulse reducing section. The wave-shaping section forms the voltage induced in a phase-winding into a rectangular wave voltage signal having a frequency. The pulse multiplying section provides a pulse signal having a predetermined number of times as many pulses as the frequency of the rectangular wave voltage signal, and the pulse reducing section provides a pulse signal having a fraction of the pulses of the pulse signal provided by the pulse multiplying section.
The frequency-pulse conversion circuit is formed by the following steps:
a step of forming a first frequency-pulse conversion circuit for outputting the pulse signal provided by the pulse multiplying section and a second frequency-pulse conversion circuit for outputting the fraction of the pulses of the pulse signal or the rectangular wave signal to the rotation speed detecting means;
a step of mounting the first frequency-pulse conversion circuit into a first controller for one of ac generators; and
a step of mounting the second frequency-pulse conversion circuit into a second controller for the other ac generator.
Therefore, the frequency provided by the multiplying section to be applied to one ac generator can be reduced by the pulse reducing section to be applied to the other ac generator easily.
In the above-described method, the pulse multiplying section preferably includes a plurality of serial pulse increase stages each of which adds one pulse formed from a leading edge of a pulse of a pulse signal inputted thereto that delays by a predetermined time, which is an ordinal number of the stagexc3x97the delay time. The method further includes a step of changing the serial pulse increase stages to form a first frequency-pulse conversion circuit and a second frequency-pulse conversion circuit, a step of mounting the first frequency-pulse conversion circuit into a first controller for one ac generator, and a step of mounting the second frequency-pulse conversion circuit into a second controller for the other ac generator.